1. Field of the Invention
The present invention relates to magnetic resonance imaging and, more particularly, to a high resolution magnetic resonance imaging system and to components thereof.
2. The Prior Art
Magnetic Resonance Imaging (MRI) has proven to be an enormously useful technology both for the detection and diagnosis of human disease as well as for research into the understanding of basic animal physiology. However, current MRI equipment has been limited by achievable signal-to-noise ratio (SNR) and by limitations in the ability to generate homogenous transmit fields for signal excitation, particularly at high magnetic field strengths.
For the acquisition of data from a nuclear magnetic resonance (NMR) signal, four separate components are required. First a static magnetic field must be generated by a permanent magnet generally of the superconducting type. Pursuant to quantum mechanics, the presence of the static magnetic field causes in a subject an energy difference between atomic spins aligned with and against this static magnetic field. The magnitude of the energy difference depends on a variety of factors, including strength of the magnetic field, size of the magnetic moments of individual atomic nuclei, and temperature. In general, a majority of the atomic spins will align with the static magnetic field and a higher energy minority of the atomic spins will align against it. When exposed to an oscillating magnetic field of proper frequency, such as is generated by an alternating current in a radio frequency (RF) coil, some of the lower energy spins aligned with the static magnetic field will be excited to the higher energy state of being aligned against the field. Once the applied transmit RF magnetic field is removed, these excited spins will decay to the lower energy state of alignment with the static magnetic field. During the decay, these spins will generate their own RF magnetic field, which can be electronically detected by the same or a different RF coil and thereby be characterized. In order to determine spatial information about the quantity and properties of the atomic nuclei of the subject, a second set of coils, gradient coils, are used to perturb the static magnetic field. By generating magnetic field gradients, current in this separate set of coils spatially changes the oscillation frequency of the atomic spins by changing the frequency of the nuclear magnetic resonance (NMR) oscillation at appropriate times during transmit and receive, and spatial information regarding the atomic spins can be decoded and converted into an image. The generation and reception of the NMR signal in the RF coil and the currents in the gradient coils are controlled by a computer system which processes the information obtained and displays it on a computer screen or printed film for human interpretation.
The advantages of using NMR are several-fold. First, information can be obtained non-invasively on a wide variety of in vitro and in vivo subjects. The lack of non-ionizing radiation is particularly attractive when images are obtained from human subjects. Second, the properties of the magnetic spins are extremely sensitive to their surrounding chemical environment. This allows a great deal of information to be determined from the magnetic resonance signal, including chemical and molecular structure of a wide variety of materials as well as the chemical and structural characteristics of animal and human tissue. By obtaining spatially dependent information regarding the NMR signal, it is possible to obtain detailed images, which not only show great anatomic detail, but which also depend on the chemical properties of tissue. This provides additional image contrast, allows improved discrimination between healthy and diseased tissue, and permits researchers to obtain previously unavailable information regarding in vivo physiologic function.
Despite the multiple advantages of MRI, one major limiting factor in the usefulness of the NMR machine is the small magnitude of the NMR signal generated by a subject""s nuclei themselves. This weak signal is easily obscured by the noise present in all electronic detection devices. The presence of this noise then limits the maximum achievable resolution or sensitivity of the NMR machine, specifically, its ability to resolve small anatomic details or to characterize time dependent changes in signal intensity, which are important for understanding of a subject""s physiology.
In principle, one can improve the sensitivity of the NMR device by increasing the strength of the static magnetic field. While this does increase signal to noise ratio (SNR), it adds problems in terms of the interaction of high frequency magnetic fields and human tissue, leading to difficulties in achieving uniform image quality and even excitation of NMR spins. Simply increasing the magnetic field strength is a very expensive option: a 3T (3-Tesla) human size magnet costs roughly five times that of a 1.5T magnet. In general, such increased cost places a premium on maximizing SNR at a given field strength.
Most of the noise in human MRI comes from the resistance associated with conductive tissue within the human body. As this resistance is roughly proportional to volume of tissue, large coils, which couple to larger volume of tissue, inherently produce lower quality images than smaller coils. While sensitivity can be improved by making smaller coils, there is a limit to this approach in that eventually the desired body part or region of interest will not fit within the coil or field of view of the coil.
One prior art method designed to increase the field of view of small coils is to use multiple coils arranged in a xe2x80x9cphased arrayxe2x80x9d (U.S. Pat. No. 4,887,039). In this method, the images from each individual coil are processed separately and then combined in such a fashion as to maximize image quality. While this is a useful strategy, it has certain limitations. First the individual coils need to be carefully oriented to minimize their respective coupling. Despite proper orientation, there always will be residual coupling between four or more coils limiting the maximum number of coils and consequently the gains in sensitivity. Furthermore, in the standard geometry feasible with surface coils, this arrangement still produces inhomogeneous images, which can complicate their interpretation for diagnostic purposes.
A second problem is the efficient and uniform excitation of the NMR spins. For most imaging sequences, a homogenous excitation of all spins is required. In general, this requires a larger coil, which then reduces the sensitivity of the system. One commonly used technique is to use a larger coil, optimized for transmit with a second coil specialized for receive. However such systems, as presently implemented, suffer from several disadvantages, particularly when used in high field systems. One disadvantage of current volume transmit coils is the inability to control the field to compensate for variations in patient size and position. While, in principle these variations can be accomplished by manually tuning the coil (see J. Thomas Vaughan, Hoby P. Hetherington, Joe O. Out, Jullie W. Pan, Gerald M. Pohost, xe2x80x9cHigh Frequency Volume Coils for Clinical NMR Imaging and Spectroscopyxe2x80x9d, Magnetic Resonance in Medicine 32:206-218 (1994)) or by using electromechanical relays to switch in additional reactive circuit elements, such methods are time consuming and subject to the variability of mechanical connections.
Conventional MRI coils come in two basic categories. (1) The simpler, the surface coil, consists of one or more conductive loops. Additional reactive circuit components, such as capacitors and inductors, are used to tune the coil and couple energy to or from it to the rest of the NMR system. Importantly, active circuit elements, such as PIN diodes, can be added to allow specialization of coil function for receive or transmit. (2) Volume coils, such as birdcage coils, consist of one or more large surface coils oriented in such a fashion as to produce a homogenous magnetic field. While such coils are in common use, the large size of these coils makes them poor receivers of NMR signal. This difficulty can be overcome by using PIN diodes to xe2x80x9cdetunexe2x80x9d the volume coil for use with a more sensitive surface coil receiver.
In particular, at high fields, the use of volume coils becomes increasing problematic. The large size of these coils required to enclose a useful area of human anatomy, such as the torso or head, leads to them becoming efficient radiators of electromagnetic energy. Moreover, the interaction of large volume coils with tissue at high frequencies leads to non-uniform magnetic fields within human tissue complicating the ability to obtain uniform spin excitation.
The following U.S. Pat. No. 5,557,247 to Vaughn, U.S. Pat. No. 4,751,464 to Bridges, U.S. Pat. No. 4,746,866 to Roschmann and U.S. Pat. No. 4,506,224 to Krause, disclose volume coils based on cavity resonators. Conductive segments within the cavity interact to form a resonant structure. While this coil can offer improved efficiency over a conventional volume coil, several disadvantages exist. First, the structure being closed can give a subject a sense of claustrophobia and make it difficult to present visual stimulation for research purposes. Second, the closed shielded nature of the coil makes it difficult to specialize for the use of transmit or receive purposes. If circuit elements are added to detune the coil, the outer cavity shield will interact with smaller coils placed with the larger cavity, impairing their performance. Additionally, the cavity shield prevents the use of the coil for specialization as a smaller coil to use with receive only function or as its use as a phased array.
The present invention is an improved NMR coil design based on the use of transmission line segments rather than conventional inductive coil elements. The use of transmission lines has several benefits. Transmission lines have a concentration of electromagnetic fields between their elements. By adjusting the distance between these conductive elements, interaction of the magnetic fields of the transmission line with an external sample can be controlled and optimized for NMR signal generation and/or detection. The presence of two conductors also decreases the inductance of each conductor. This minimizes the electric fields associated with the conductors, which is advantageous since these electric fields can be associated with dielectric tissue losses which decrease coil efficiency and sensitivity. Moreover, the inherent shielded nature of transmission lines decreases the radiation of electromagnetic energy from the NMR coil, improving coil efficiency and sensitivity over conventional NMR coil design. The shielded nature of a transmission line also decreases the interaction or coupling between coil elements. This can be advantageous since under proper conditions, coil elements can operate with minimal interaction. This allows a large single coil structure to operate as multiple smaller individual coils. With proper combinations, these separate coils can be combined in such a way to optimize NMR signal generation and/or reception. In particular, by combining signals from individual coil elements, spatial information may be decoded regarding the NMR signal, increasing the sensitivity and speed of data acquisition for both high field and low field NMR systems.
The coil consists of N transmission line segments distributed in a circular, elliptical, or other geometrical arrangement. Each transmission line element is comprised of two or more individual conductors with or without additional lumped or distributed capacitive or inductive circuit components. In general, each transmission line element couples to the others through mutual inductance and capacitive coupling. Additional lumped or distributed inductive or capacitive elements may be placed between the transmission line segments to alter this coupling. The combined influences of the interaction between these elements gives rise to frequency dependent relations between the currents and voltages present on individual transmission line elements. By changing the individual circuit components and transmission line geometry, a given current distribution can be obtained on the transmission line elements at a given frequency. The magnetic field arising from the currents on each element add through superposition and create a given magnetic field configuration for use in either or both the generation and detection of the NMR signal.
In particular, with placement of properly valued reactive components between individual transmission line elements, mutual coupling between elements can be minimized. This allows the resonant structure of the N transmission line segment to become degenerate and allows the currents on each element to be relatively independent. This has the advantage for NMR signal generation in that the currents on each element can be individually controlled at will to generate a excitation field of a desired spatial and phase characteristic. Additionally in such an degenerate mode arrangement, received signals from each element are independent and can then be combined in such a way to optimize image homogeneity, sensitivity, or other desired parameters.
In order for the transmission line structure to be useful, energy needs to be transferred into the coil during signal generation and out of the coil during signal reception. This can be accomplished by inductively or capacitively coupling one or more circuit elements to one or more RF power amplifiers and/or RF receivers. This coupling can be adjusted to allow an arbitrary impedance of such equipment to be matched to the currents and voltages found in the transmission line structure. In particular, the phases of the current in two or more transmission line elements can be offset as to create elliptically polarized magnetic fields for improved efficiency in the generation and/or detection of the nuclear magnetic resonance signal.
In addition to passive components, active circuit elements such as diodes (either regular or PIN) can be added to this structure. With diodes, the tuning of individual transmission line elements or their mutual coupling can be changed in order to modify the current distribution and element impedance of the transmission line segments. When used with one or more additional coils (which may be a combination of transmission line structures or conventional NMR coils), these diodes can be arranged so that during transmit or receive functions, one coil has a desired magnetic field configuration while the other coil presents a high impedance so as not to interfere with the magnetic fields of the first coil. In this manner, each of the two or more coils can be optimized for either transmit or receive, resulting in improved generation and detection of the NMR signal.
Other active circuit elements can be added to the transmission line structure such as vacuum tubes or transistors (including but not limited to conventional bipolar transistors, field effect transistors, gallium arsenide field effect transistors, high electron mobility transistors, pseudomorphic high electron mobility transistors, or heterojunction bipolar transistors). These transistors can be used to provide amplification of either the transmit energy needed in the generation of the nuclear magnetic signal or the small magnitude received energy from the NMR spin decay. In this way, signal losses arising from matching circuits and connecting cables are minimized, leading to improved coil efficiency. If the coil is designed for both transmit and receive functions, diodes may be included to change the coupling between these active amplifier circuits and individual transmission line elements. In this manner, transistors designed for low-noise signal amplification are not damaged by the high element currents during the transmit function and transistors circuits designed for power amplification do not add noise during signal reception.
The addition of active vacuum tube or transistor circuits can provide additional advantages. With proper design, these circuits can present impedance mismatches to the transmission line structure while simultaneously preserving adequate amplifier function. These impedance mismatches can be used to change or minimize coupling between individual transmission line elements, allowing the elements to be decoupled and be relatively independent of each other. During transmit, this has the advantage that individual element currents can be changed electronically in magnitude or phase so as to modify the desired magnetic field for optimal transmit excitation without requiring change or variation of passive circuit elements. This is particularly advantageous at high frequencies where dielectric resonances in human tissue require non-uniform magnetic fields for uniform spin excitation. Additionally, during receive, decoupling of the currents on transmission line elements allows each element to function as a separate signal detector. By combining the signals from these elements electronically, either directly after amplification or at a later stage such as after image reconstruction, these signals can be added in a way such that sensitivity is maximized for one or more areas of interest. In particular, the spatially dependent information from each element can be combined after image reconstruction in such a manner that sensitivity is maximized at each point in an image.
Moreover, the geometric arrangement of the individual transmission line elements can be used to decode spatial information regarding the detected NMR signal. By decoding spatial information from individual coil elements, the steps required for the acquisition of an NMR image can be reduced, allowing the imaging process to be completed in less time.
The illustrated embodiments of the present invention demonstrate an actively decoupled transmission line resonator for use as a transmit coil in conjunction with surface coil receivers, as well as use of a transmission line structure as a receive array coil.
Other objects of the present invention will become apparent in light of the following drawings and detailed description of the invention.